1. Field of the Invention
This invention relates to the field of hard disk drives. More particularly, this invention relates to the field of a multilayer bond pad for hard disk drive suspensions.
2. Description of Related Art
A hard disc drive (HDD) unit generally uses a spinning storage medium (e.g., a disc or platter) to store data. A read-write head is positioned in close proximity to the spinning storage medium by a Head Stack Assembly (HSA). Mounted on the HSA, a suspension assembly commonly includes a base plate, a load beam, and a flexure trace gimbal to which a slider is mounted. The suspension is mounted to a support arm, also called an actuator arm or a suspension arm. The slider supports the read-write transducer head element. The load beam is generally composed of an actuator mounting section, a spring region, and a rigid region. The spring region gives the suspension a spring force or preload counteracting the aerodynamic lift force created by the spinning medium during reading or writing. A gimbal is mounted at the distal end of the load beam and supports the slider allowing the head to have pitch and roll movement in order to follow the irregularities of the disc surface.
FIG. 1 is a simplified general diagram of a prior art disc drive apparatus 100. Apparatus 100 includes at least one disc 101 (e.g., one, two, three, or more discs), at least one actuator arm or support arm 103 (e.g., one, two, three, or more actuator arms), and at least one suspension assembly 105 (e.g., one, two, three, or more suspension assemblies). Each suspension assembly is composed of a load beam 107 and a flexure trace gimbal assembly 109. The suspension assembly, with trace gimbal assembly and read/write transducer head, may be referred to as a Head Gimbal Assembly (HGA).
Suspensions for hard disk drives and other devices such as optical disk drives include a multi-layer circuit that carries signals between the read/write head, and possibly other parts of the suspension such as one or more microactuators located on the suspension, to the rest of the circuitry within the disk drive. Currently, it is common for suspension circuits to employ vias to form electrical connections between different layers. Vias may be used to connect trace layers such as in interleaved traces for low impedance/high bandwidth interconnects, to connect signal traces to bond pads, to connect a signal trace to a grounded portion of the stainless steel suspension, and to connect other circuit components. Typically, a via is formed in a dielectric layer by making a hole in the layer using laser drilling or chemical etching, and then filling the via hole using either conductive epoxy or electroplating. For example, U.S. Pat. No. 7,272,889 issued to Aounuma et al. purports to disclose one technique for creating such vias. Vias typically range from 20-80 μm in size. When vias are used with a capture pad, the capture pads are typically from 60-120 μm in size.
FIG. 2 is a simplified sectional representation of a prior art via 202. A hole 204 is drilled such as by laser drilling into a layer of dielectric material 206 (e.g., polyimide), the hole 204 extending down to the metalized surface that defines a first metal layer 208 below the dielectric material 206. The via 202 could be either a through-via, which is created by mechanical or laser drilling, all the way down through a first metal layer, or it could be a blind via, which is a via that extends only down to rather than through the first metal layer. Blind vias can be created either by precise numerically controlled (NC) drills with precise depth control or preferably by lasers having specific wavelengths to selectively ablate a first metal layer and then the internal dielectric, stopping at a second metal layer.
In some via manufacturing processes, the second metal layer 210 shown in FIG. 2 is added over and into the hole 204 by electrodeposition. In this way, the second metal layer 210 extends down into the hole 204 thereby defining the via 202. A protective and electrically nonconductive cover coat 212 is then applied to the laminate assembly. This via forming process requires that a photoresist mask that is used to create the second metal layer 210 and its circuit trace pattern be closely aligned with the circuit pattern on the first metal layer 208.
Other types of manufacturing processes (other than electrodepositing) may be used to create vias. For example, a hole could instead be filled with some other conductive material such as a conductive epoxy to form a conductive via.
FIG. 3 is an oblique view of an example of a prior art suspension circuit 302 in the area around a read-write head slider 308, illustrating the use of the via 202 to create an interconnection from one metal layer to another layer in the area of the suspension's slider bond pads 310. Read+ and Read− differential signal pair traces 304A and 304B, and Write+ and the Write− differential signal pair traces 306A and 306B, are respectively routed on top of one another most of their distances on the suspension circuit 302 to reduce the impedance of each signal transmission line and to reduce the susceptibility of each signal pair to cross talk. One polarity of a given pair is routed up at a corresponding one of the vias 202 to the top layer where the other polarity is located near the head slider 308 to connect thereto. All of the slider bond pads 310 are therefore located at the same level for electrical connection to the slider 308, such as by solder ball bonding.
The vias 202 shown have via pads of sufficient size so that they can be manufactured in volume without defects due to layer misregistration. The illustration shows a design that is currently available from circuit suppliers.
The structure 312 on the very distal end of the circuit 302 containing the three copper rings provides a ground path to a stainless steel suspension structure below to protect the slider 308, which is sensitive to electrostatic discharge (ESD) damage. The rings are used as reference points by machine vision assembly equipment (not shown) both at the suspension manufacturing level as well as at the head stack assembly (HSA) level.